1. Field of the Invention
This invention relates to a radiation image recording apparatus for storing a radiation image of an object on a stimulable phosphor sheet. This invention also relates to a radiation image recording and read-out apparatus for storing a radiation image of an object on a stimulable phosphor sheet and thereafter photoelectrically reading out the radiation image from the stimulable phosphor sheet. This invention further relates to a cassette for housing at least a single stimulable phosphor sheet during an operation for storing a radiation image on the stimulable phosphor sheet. This invention particularly relates to a radiation image recording apparatus, a radiation image recording and read-out apparatus, and a cassette, wherein radiation images to be subjected to energy subtraction processing and radiation images to be subjected to superposition processing are stored on stimulable phosphor sheets easily and selectively.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
A radiation image recording apparatus, which utilizes a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet), has been disclosed in, for example, Japanese Unexamined Patent Publication No. 59(1984)-102227. With the disclosed radiation image recording apparatus, a stimulable phosphor sheet is housed at a position for image recording and exposed to radiation carrying image information, and a radiation image of the object is thereby stored on the stimulable phosphor sheet.
Also, cassettes for housing a stimulable phosphor sheet therein have heretofore been known. The cassette is constituted of a box member provided with an opening, through which a stimulable phosphor sheet is to be fed into and out of the box member, and a cover member, which opens and closes the opening. The cassette houses the stimulable phosphor sheet in the box member and prevents the stimulable phosphor sheet, on which no radiation image has been stored, from being exposed to light before the stimulable phosphor sheet is used during an operation for recording a radiation image. Also, the cassette prevents the stimulable phosphor sheet, on which a radiation image has been stored, from being exposed to light before the stimulable phosphor sheet is subjected to an operation for reading out the radiation image.
Further, as disclosed in, for example, U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, the applicant proposed various radiation image recording and reproducing systems, which utilize stimulable phosphor sheets. Specifically, a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. A laser beam is then modulated with the image signal, and a photosensitive recording material or a thermosensitive recording material is two-dimensionally scanned with the modulated laser beam. In this manner, the radiation image is reproduced as a visible image on the recording material.
With the proposed radiation image recording and reproducing systems, radiation images can be recorded even when the energy intensity of the radiation, to which the stimulable phosphor sheet is exposed, varies over a wide range. Also, visible radiation images can be obtained which have good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Also, the applicant proposed radiation image recording and read-out apparatuses, which efficiently process stimulable phosphor sheets, in, for example, U.S. Pat. No. 4,851,679 and Japanese Unexamined Patent Publication No. 3(1991)-238441. The proposed radiation image recording and read-out apparatuses comprise: i) a circulation and conveyance means for conveying at least a single stimulable phosphor sheet along a circulation path, ii) an image recording section, which is located in the circulation path and in which a radiation image of an object is stored on the stimulable phosphor sheet, iii) an image read-out section, which is located in the circulation path and in which the radiation image having been stored on the stimulable phosphor sheet is read out from the stimulable phosphor sheet, and iv) an erasing section, which is located in the circulation path and in which energy remaining on the stimulable phosphor sheet is released after the radiation image has been read out therefrom.
Also, as disclosed in, for example, U.S. Pat. No. 4,855,598, techniques for carrying out energy subtraction processing on radiation images have heretofore been known.
When energy subtraction processing is to be carried out, a plurality of radiation images are recorded under different conditions of energy absorption characteristics with respect to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays. The radiation images are then photoelectrically read out, and digital image signals which represent the radiation images are thereby obtained. The image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images, the specific structure having energy absorption characteristics different from those of the other structures of the object. With the energy subtraction processing method, a plurality of digital image signals are subtracted from each other, and a difference signal is thereby obtained. The radiation image of a specific structure of the object can then be reproduced from the difference signal.
Specifically, with the energy subtraction processing method, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of at least two radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, at least two radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, the image signals representing at least two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process, and the image of the specific structure of the object is thereby extracted.
In cases where energy subtraction processing is to be carried out by using stimulable phosphor sheets, radiation images may be stored on at least two stimulable phosphor sheets so that the parts of the radiation images corresponding to a specific structure of an object may be different in the at least two radiation images. Specifically, for example, a plate-shaped filter, which may be constituted of a metal, or the like, and which is capable of absorbing low energy components of radiation, may be located between two stimulable phosphor sheets, which are placed one upon the other. The two stimulable phosphor sheets, which are placed one upon the other with the filter intervening therebetween, may then be simultaneously exposed to radiation carrying image information of an object.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image may be enhanced. Practically, the detection performance is adversely affected by various noises.
Specifically, in radiation image recording systems using stimulable phosphor sheets, it has been found that the noises described below occur during the step for recording a radiation image on a stimulable phosphor sheet and reading out the radiation image therefrom.
(1) A quantum noise of radiation produced by a radiation source. PA1 (2) A noise due to nonuniformity in how a stimulable phosphor coated on the stimulable phosphor sheet is distributed or how stimulable phosphor grains are distributed on the stimulable phosphor sheet. PA1 (3) A noise of stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. PA1 (4) An electric noise in the means for detecting light, which is emitted by the stimulable phosphor sheet, and converting it into an electric signal. PA1 (5) A noise of light emitted by the stimulable phosphor sheet. PA1 wherein the sheet housing section is divided into at least three sheet housing compartments by: PA1 i) a circulation and conveyance means for conveying at least a single stimulable phosphor sheet, which is capable of storing a radiation image thereon, along a predetermined circulation path, PA1 ii) an image recording section, which is located in the circulation path and provided with a sheet housing section for supporting and housing the stimulable phosphor sheet therein, and in which the stimulable phosphor sheet is exposed to radiation carrying image information of an object, a radiation image of the object being thereby stored on the stimulable phosphor sheet, PA1 iii) an image read-out section, which is located in the circulation path and provided with: PA1 iv) an erasing section, which is located in the circulation path and in which energy remaining on the stimulable phosphor sheet after the image signal has been obtained therefrom in the image read-out section is released before a next radiation image is stored on the stimulable phosphor sheet, PA1 wherein the sheet housing section is divided into at least three sheet housing compartments by: PA1 each of the radiation energy distribution separating filter, the radiation transmitting member, and at least either one of a front side member, which constitutes an end face of the sheet housing section on the side close to the object, and a back side member, which constitutes an end face of the sheet housing section on the side remote from the object, is movable between an open position, which at least selectively opens each of the sheet housing compartments in order to allow the stimulable phosphor sheet to be fed into and out of each of the sheet housing compartments, and a closed position, which closes each of the sheet housing compartments in order for the radiation image to be stored on the stimulable phosphor sheet. PA1 wherein a region inside of the box member is divided into at least three sheet housing compartments by:
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. Specifically, for example, two stimulable phosphor sheets, which are placed one upon the other, are simultaneously exposed to radiation, which has passed through an object. In this manner, two radiation images are stored on the stimulable phosphor sheets with the radiation having approximately identical energy distributions. Thereafter, the radiation images are photoelectrically read out from the stimulable phosphor sheets, and digital image signals representing the radiation images are thereby obtained. The digital image signals are weighted appropriately, and the image signal components of the weighted digital image signals, which represent corresponding picture elements in the radiation images, are then added to each other or averaged.
Specifically, in general, various noises described above exhibit different distributions for different radiation images stored on the stimulable phosphor sheets. When the image signals detected from the stimulable phosphor sheets are superposed one upon another, the noises can be averaged. Therefore, the noises become imperceptible in a superposition image, which is obtained from superposition processing. In this manner, the detection performance of the radiation image can be enhanced.
As described above, when radiation images for energy subtraction processing are to be obtained, a filter capable of absorbing low energy components of radiation should be located between two stimulable phosphor sheets, and the combination of the filter and the stimulable phosphor sheets should be set at the position for radiation image recording. Also, when radiation images for superposition processing are to be obtained, two stimulable phosphor sheets should be placed one upon the other, and the combination of the two stimulable phosphor sheets should be set at the position for radiation image recording. However, no apparatus for automatically carrying out such setting operations is available, and considerable time and labor are required to carry out the setting operations manually.